I use a Tualatin 1.4GHz with Win2k SP4 and it's still slow on most web sites. It runs Firefox 5 with noScript, but as soon as some scripts have to be enabled, it gets slow. It's limited to 512MB RAM but I don't think that's the most pressing problem. The onboard graphics are basic Intel junk. Flash acceleration would probably help a lot, but I don't think even an upgraded card will allow Flash acceleration without SSE2.
Come to think of it, I wonder whether it's feasible to bypass the SSE2 requirement with any open source video drivers on Linux or BSD.
I do use that Win2k Tualatin machine daily but only for a few specific sites. It's just a matter of convenience with where that machine is located, and it's compact size. If I have a little extra time, I'll sometimes get on VOGONS with it so I'm grateful this forum works well with older machines.
It's amazing how much SSE2 has become a curse for older CPUs. When it was introduced, I just figured it was another minor tweak to SIMD instructions which had already been around for a while. I never would have expected it to become such a significant break point in terms of compatibility and performance.